Transformer core



TRANSFORMER CORE Filed Nov. 25, 1947 Fig.2.

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Patented May 22, 1951 2,553,554 TRANSFORMER 001m Roy H. Dierstein, FortWayne, Ind., as'signor to General. Electric Company, a corporation ofNew York Application November 25, 1947', Serial ndvs'aosa 7 Glaim's.(c1. 17-1-47 7) This invention relates to magnetic cores, and moreparticularly to improvements in cores for stationary inductionapparatus.

In many types of magnetic cores, it is necessary to provide gaps in themagnetic circuit which are either complete breaks or gaps or else areso-called bridged gaps, in which in effect, there is a gap which ismagnetically paralleled by a restricted section of the core which has ahigher flux density than the rest of the core. Such gaps are used tocontrol the exciting current of induction apparatus or the distributionof the flux in various parts of the core, and due to magnetic saturationin the bridging parts of bridged gaps, to control also the rate ofchange of flux in certain parts of the core relative to other partsthereof, so as to provide certain desirable voltage changes in thewindings linking various parts of the core.

In accordance with this invention, there is provided a novel and simplecore structure in which all of the gaps are formed in the samestructural part of the core. Furthermore, in the preferred forms of theinvention, this part is a relatively straight large cross sectionwinding leg and the gaps are formed by providing notches or recesses inthis part. In this way, relatively heavy and strong dies can be used'forforming the notches at the same time that the relatively largelamination punchings which form this part are stamped out. This makesfor much greater accuracy in'the dimensioning of the gaps. Also, as thelaminations of the'core part which iorm the gaps can be made with arelatively rugged die, because there are no small pointed parts or partsof similar shape, the die wear is greatly reduced.

An object of the invention is to provide a new and improved magneticcore for stationary apparatus.

Another object of the inventionis toprovide a new and improved core forhigh reactance transformers which have one or more magnetic gaps.

An additional object of the invention is to provide a new and improvedshell-type transformer core having a plurality of magnetic gaps, all ofwhich are formed in a single, generally straight central winding legmember.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing and its scope will bepointed out in the appended claims.

In the drawing, Fig. 1 is a plan view of acore for a high reactanoetransformer type two lamp lead lag fluorescent ballast, which embodiesthe invention, Fig. 2 is a modification cf Fig. l .in K

which the relative positions of the bridge and gap portions of thebridged-gaps have been reversed, Fig. 3 is a view similar to Fig. 1 of acore, suitable for use in high reactance transformer two lead lampfluorescent ballasts, which embodies the invention, and Fig. 4 is 'amodification in which the bridging portions of the bridged gaps of Fig.3 have been reversed.

Referring now to the drawing, and more particularly to Fig. l, the corecomprises a generally straight winding leg I and oppositely disposedmulti-leg yoke members 2. It Will, of course, be understood that themembers I and 2 are laminated and the shapes of these members shown inthe-drawing. may be taken to correspond With the actual shapes of thelamination pieces, the members then consisting of a plurality of piecesof the same shape stacked one above the other. The yoke members have endlegs 3 and 4 and have intermediate leakage flux legs 5 and 6. In Fig. 1,the legs are all of the same length and their ends are square cut.

The center section of the winding leg I, which is between the magneticshunt legs 5 and 6. is adapted to receive a winding which may, forexample, be the primary or exciting winding of a high reactancetransformer. The section of the winding leg I, which is between the yokelegs 4 and 5, is adapted to receive another transformer winding whichmay be a secondary winding, of either the insulated or extended.autotransfcrmer type, and like vise, the end section of the winding legbetween the yoke 1egs-3 and 6 is adapted to receive another suchsecondary winding. Such windings may be conventional, and consequently,they are not shown. is well known in two lamp ballasts for fluorescentlamps, one secondary winding is often operated with leading currentwhich is produced by a series capacitor in the leading lamp circuit, andthe other lamp and secondary winding is-operated with lagging currentwhich isproduced by the transformers leakage reactaince. In this manner,the overall power factor of the device is relatively high.

It will be observed that the yoke leg Bis substantially narrower thanthe yoke leg. 6. This is because theleakage fluxwhich traver'sesthesetwo legs is substantially different in magnitude, due to the differencein the phase ofthe leading and lagging secondary fluxes with relation totheprimary or'exciting flux. Thus, if the secondary winding; whichnormally occupies the space between the legs 4 and 5 carries leading 3current, then the flux in that part of the winding leg is more or lessin phase with the exciting flux so that the vector difference betweenthese two fluxes which is the only flux which flows in the yoke legs 5will be relatively small. Likewise, if the portion of the winding legwhich is between yoke legs 3 and 6 carries a lagging current secondarywinding, then the flux produced by that winding is more or less in phaseopposition to the exciting flux in the central part of the winding legI, and consequently, the vector difference between those two fluxes isrelatively large and it is thisvector resultant flux which flows in theyoke legs 6. Consequently, in order to have approximately equal fluxdensity in the two leakage path yoke legs 5 and 6, the latter has asubstantially greater area than the former.

In order to prevent the leakage flux paths from having too low areluctance, it is usually necessary to provide them with a highreluctance section, such as an air gap and it has also been found thatdesirable results are obtained when the portion of the core which istraversed b the leading secondary flux is also provided with a highreluctance section, preferably a bridged gap. In Fig. 1, these variousgaps are all obtained by properly shaping the winding leg punchings.Thus, it will be observed that there are depressions or notches l in thesides of the winding leg in the vicinity of the ends of the yoke legs 5,so as to form air gaps. Similarly, there are notches or depressions 8 inthe winding leg i in the vicinity of the ends of th yoke legs 5 so as toform air gaps therebetween. It will further be observed that the notchesl and 8 are in substantial alignment respectively with the center linesof the yoke legs 6 and 5 so that the gaps are full and complete gaps.

Notches 9 are also cut out of the left hand end of the winding leg i,but these notches are not as wide as the end yoke legs 4 and they areoffset toward the left as viewed in the drawing, from the center linesof the yoke legs 4 so that the gaps which are formed by thisconstruction are bridged by shoulder portions iii on the winding legwhich makes butt contact with part of the ends of the yoke legs 4. Inthis manner, bridged gaps are formed. The inclusion of bridged gaps inthe lead end of the magnetic core of a two lamp lead-lag hightransformer ballast for discharge lamps such as fluorescent lamps isdisclosed and broadly claimed in an application Serial No. 731,559,filed February 28, 1947 in the name of Harold W. Lord and assigned tothe same assignee. The right-hand end of the winding leg does not haveits sides notched, and consequently, those sides make low reluctancebutt contact with the ends of the yoke legs 3.

By means of this construction, the dimensions of the gaps can be veryaccurately maintained, and as these dimensions are determined by theshape of a relatively rugged die which does not have to punch out smallpointed projections on the laminations the life of the die is verymaterially increased and the control of the gap proportions is greatlyimproved.

In the modification shown in Fig. 2, the winding leg I differs from thewinding leg I in Fig. 1, in that the relative positions of the endnotches and shoulders which form the bridged gaps at the left-hand ofthe core, have been reversed. Thus, there is a notch 9' in the windingleg I which is offset inwardly or toward the center of the core from thecenter line of the yoke legs 4, so as to form what may'be considered asshoulders I 0 on the winding leg I which make butt contact with theouter parts of the ends of the yoke legs 4 so as to form the bridges ofthe bridged gaps. An advantage of this latter construction is that themagnetic bridge of the gaps being on the outside, they tend to retainthe flux in the core and prevent it from leaking out into adjoiningareas and metal surfaces such as a metal case for the device. Thus, inFig. 1 the flux which goes through the air gap between the notch 9 andthe yoke leg 4, after the shoulder I0 is saturated, tends to fringeoutwardly and stray into adjoining metal parts and thus causeundesirable heating of those parts. In Fig. 2 any flux which tends tostray or fringe out from the air gap between the notch 9' and the endlegs 4 will fringe inwardly toward the center of the core.

Fig. 3 is a modified core construction which is suitable for use in highreactance transformer ballasts, which supply two leading currentdischarge lamp circuits. This construction is symmetrical because thetwo end parts of the core carry similar magnitude fluxes having similarleading phase relationship with respect to the exciting flux whichtraverses the center part of the core. The duplicate yokes 2 are similarto the yokes 2 in Figs. 1 and 2, except that the two intermediateleakage flux yoke legs are of equal width. These are shown at H in Fig.3. The winding leg i differs from the winding leg I in Fig. 1 in thatthere is a bridged gap 9-40 on opposite sides of each of its two ends.

Fig. 4 bears the same relation to Fig. 3 that Fig. 2 does to Fig. 1 andit differs from Fig. 3 in that the relative positions of the notches 9and the bridges iii are reversed so that the bridges are on the outside.of the core and tend to act as shields for retaining the flux in thecore and preventing it from fringing out and entering adjoining metalparts.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications can be made without departing from the invention and,therefore, it is aimed in the appended claims to cover all such changesand modifications as fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A magnetic core for stationary induction apparatus, comprising, incombination, a yoke member having a plurality of sidewise extending legsincluding end legs and at least one intermediate leg, and a winding legcontiguous to the ends of said yoke legs, said winding leg having anotch in the vicinity of the end of one of said yoke end legs forproviding a magnetic gap in said core, said notch being oifset from theend of the winding leg and from the center line of said yoke end leg sothat said gap is a bridged gap with the bridge part thereof toward theoutside of said core with respect to the gap part thereof.

2. A core for a high reactance transformer type two lamp fluorescentballast, comprising, in combination, a pair of four-legged yoke memberswhose inner two legs are of substantially different width, said yokemembers being disposed with their legs extending toward each other, agenerally straight winding leg between said yoke members and lyingacross the ends of their legs, notches on opposite sides of the end ofthe winding leg which is adjacent the end yoke legs which are nearestthe narrower of the two intermediate yoke legs, said notches beingoiTset from the center lines of the adjacent end yoke legs so as to formbridged magnetic gaps.

3. A core for a high reactance transformer type two lamp fluorescentballast comprising, in combination, a pair of four-legged yoke memberswhose inner two legs are of substantially difierent width, said yokemembers being disposed with their legs extending toward each other, agenerally straight winding leg between said yoke members and lyingacross the ends of their legs, notches on opposite sides of the end ofthe winding leg which is adjacent the end yoke legs which are nearestthe narrower of the two intermediate yoke legs, said notches beingoffset from the center lines of the adjacent end yoke legs so as to formbridged magnetic gaps, said notches being offset toward the middle ofsaid winding leg whereby the bridge portions of said bridge gaps aretoward the outside of said core.

4. A core of a high reactance transformer ballast for two electricdischarge lamps comprising in combination, a pair of similar four leggedyoke members all of whose legs are of equal length with square ends,said yoke members being disposed opposite each other with their legsextending toward each other, a straight central winding leg memberdisposed between said yoke members and lying across the ends of theirlegs, said members all being of equal length and being permanentlyaligned crosswise with respect to each other, notches in opposite sidesof said straight winding leg member opposite to and coextensive with thewidth of the two intermediate legs of each yoke member so as to providefull magnetic gaps between said intermediate yoke legs and said windingleg member, the ends of the outer legs of each yoke member being incontact with thesides of said straight winding leg member near the endsthereof in order to make the length of said gaps equal to the depth ofsaid notches, and additional notches in opposite sides of said straightwinding leg member near at least one end thereof, said additionalnotches being partially ofiset with respect to the ends of at least onepair of said outer yoke legs so as to form bridged magnetic gaps offixed dimensions whose bridge parts determine the length of the gapsassociated with said intermediate legs.

5. A core as defined in claim 4 in which the offset of said additionalnotches is toward the outer end of said winding leg member so that thebridge part of the bridged gaps is inside of the core with respect to thgap portion thereof.

6. A core as defined in claim 4 in which said additional notches are onopposite sides of both ends of said winding leg member.

7. A core as defined in claim 4 in which said additional notches are onopposite sides of both ends of said winding leg member and the offset ofsaid additional notches is toward the center of said winding leg memberso that the bridge part of the bridged gaps is toward the outside ofsaid core with respect to the gap portion thereof.

ROY H. DIERSTEIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,895,231 Pearson et al. Jan. 24,1933 2,230,945 Hansell Feb. 4, 1941 2,333,995 Gaynor Nov. 9, 19432,382,638 Keiser et a1 Aug, 14, 1945

